This invention relates to thermoplastic shipping bags and in particular to bags used for packaging fine powders of less than 100 microns.
Plastics shipping bags are suitable for the packaging, transportation and storage of a wide variety of products in granular, bead, or pellet form. However, there are many products such as cement, clays, powdered coal and pigments, for instance, which cannot be readily packaged in plastic bags because of the inability of plastic films to release enough of the air which has become entrained inside the package during the high-speed filling operation. Plastic bags are known having perforations directly in their walls to provide the necessary air release. However, this can result in excessive environmental contamination and/or product loss from the package. This means that for fine powdered products having a particle size in the range of 10 microns or less, resort is generally had to the use of paper or paper-like packaging materials for generally acceptable finished packages. Paper bags are commonly used for these applications with the inner ply of the paper supplying the necessary filtering action. Such packaging materials, however, are limited by the end use of these packaged goods in several ways.
One drawback is that paper bags are extremely sensitive to environmental extremes and require special care in high humidity or low temperature conditions. Also, paper fibre contamination may result when the package's contents are emptied into sensitive chemical mixtures. Further, with the growing concern for environmental safety in the workplace, there is an increasing need for packages which can be added to, and thermally or mechanically dispersed within, industrial processes. Multi-wall paper bags, and, to some degree, plastic bags using contaminating adhesives or glues are generally incompatible, particularly in the plastics and rubber industry.
Some of these problems can be overcome by using a plastic bag made from a spun bonded plastic in the form of a mat of compressed thermoplastic fibres, such as TYVEK.TM., which has high strength characteristics while still retaining the air permeable characteristics of paper. Alternatively, a plastic film bag lined with paper achieves the same results. However, these bags have the disadvantage of high cost and, like paper bags, they have the further disadvantage of being unsuitable for use in applications where the bag as well as the product is thrown into rubber or plastic mixes where the bag is expected to mix and incorporate as part of the finished product.
One particularly useful type of plastics shipping bag is that known as a valved bag, which is generally used for packaging granular materials such as fertilizers and polymer resins. One such embodiment is described in our U.S. Pat. No. 3,833,166. These bags possess the important commercial advantage of being easily filled through a valve structure with the self-closing of this valve structure after filling. When filled and stored with the valve in the down or sealed position the effectiveness of the valve is so good that entrapped air is difficult to expel from the bags. To avoid the problem it is common practise to place a row of 10 or so 0.6 mm diameter perforations down each side of the bag to allow the air to escape. This is satisfactory for coarse granular product, but for fine powder materials of a practice size of less than 100 microns this practise is unsatisfactory since the product can readily leak through the holes in unacceptable amounts. This disadvantage is somewhat overcome by offsetting inner perforated layers of the bag from the outer perforated layers and trapping any escaping materials between the inner and outer layers of the bag while readily allowing the air to escape. However, the small number of perforated holes can somtimes clog, resulting in substantial air retention. For example, by using this system of offset perforations, typically with perforations at 2.5 cm centres over the entire body of the bag, product with particle size of down to 1.0 micron may be packaged. However, with powders of such small particle size, the product will flow into these few holes forming pyramidal plugs which prevent further flow of air from the bag.